KR20070015366A - Display device for volumetric imaging using a birefringent optical path length adjuster - Google Patents

Abstract

A display device for generating a three- dimensional volumetric image incorporates an optical path length adjuster enables electro-optical control of a physical path length between a display panel and a focusing element, to generate a virtual image within a defined imaging volume. The adjuster varies an optical path length between an input optical path and an output optical path and includes: a first polarisation switch for selecting a polarisation state for an input beam on the input optical path and an optical element having birefringent properties and thereby defining at least two possible effective optical paths of different lengths therethrough, for passing the input beam along a selected one of said at least two possible optical paths according to the selected polarisation state of the input beam and for providing an output beam of light, on said optical output path, that has travelled along the selected optical path. ® KIPO & WIPO 2007

Description

DISPLAY DEVICE FOR VOLUMETRIC IMAGING USING A BIREFRINGENT OPTICAL PATH LENGTH ADJUSTER}

The present invention relates to a method and apparatus for adjusting the optical path length between two optical elements. In particular, although not exclusively, the present invention relates to the adjustment of the optical path length in a three-dimensional display device that produces a virtual image within a defined imaging volume.

Three-dimensional images can be generated in several ways. For example, two images that can be uniquely observed through the eyes of each viewer in a stereoscopic display can be viewed simultaneously or time-multiplexed. Images are selected through special glasses or goggles worn by the viewer. In the former case, special glasses may be equipped with a polaroid lens. In the latter case, the goggles may be equipped with an electronically controlled shutter. These types of displays are relatively easy to make and have a low data-rate. However, using special viewing glasses is inconvenient and can cause inconvenience among viewers due to lack of parallax in operation.

More realistic three-dimensional representations can be generated using auto-stereoscopic displays. In this type of display, every pixel emits light of different intensity in different viewing directions. The number of viewing directions should be large enough so that each viewer's eyes can see different images. These types of displays show actual parallax of operation; If the viewer's head moves, the field of view changes accordingly.

Most of these types of displays are technically difficult to implement in practice. Several suggestions can be found in the literature, see for example US 5,969850. The advantage of these displays is that multiple viewers can watch a single 3D television display, for example, without special viewing glasses, and each viewer can see a realistic three-dimensional image including parallax and vision.

Another type of 3D display is the volumetric display described at http://www.cs.berkley.edu/jfc/MURI/LC-display. In volumetric displays, the point of the image display volume emits light. In this way, an image of a three-dimensional object can be generated. The disadvantage of this technique is that it is closed, that is, it cannot block light at a point hidden by another object. Thus, all displayed objects are transparent. In principle, this problem can be overcome by video-processing and possibly by tracking the position of the viewer's head or eyes.

A known embodiment of a volumetric display is shown in FIG. 1. The display consists of a transparent crystal 10 scanned by two lasers 11, 12 (or more). At the intersecting position 15 of the laser beams 13, 14, light 16 can be produced by up-conversion, with a lower photon emission at higher energy. Occurs through absorption of multiple photons of energy (ie from a combined laser beam). This type of display is expensive and complex. Special crystals 10 and two scanning lasers 11 and 12 are required. In addition, up-conversion is not a very efficient process.

An alternative embodiment of the volumetric display 20 is shown in FIG. 2. The device uses a material that can be switched between transparency and diffusion, such as Polymer Dispersed Liquid Crystal (PDLC) or Liquid Crystal Gel (LC-gel). In the three-dimensional grating volume 21, the cell 22 can be switched between these two states. In general, the volume 21 is illuminated from one direction. In the figure, the illumination source 23 is located below the grid volume. When cell 22 is switched to the diffuse state, light 24 is dispersed in all directions.

One object of the present invention is to provide a volumetric three-dimensional image display device that overcomes some or all of the problems associated with prior art devices.

It is another object of the present invention to provide a device suitable for adjusting the optical path length between two optical elements inside a volumetric three-dimensional image display device.

Another object of the present invention is to provide an optical path length adjuster for changing the optical path length between the input optical path and the output optical path.

Some or all of these objects can be achieved by the embodiments of the present invention described herein.

According to one aspect, the present invention provides a display device for generating a three-dimensional volumetric image,

A two-dimensional image display panel for generating a two-dimensional image;

A first focusing element for projecting the two-dimensional image to a virtual image in an imaging volume; And

Means for changing the effective light path length between the display panel and the projecting first focusing element to change the position of the virtual image in the imaging volume, wherein the means for changing the effective light path length includes an input light path and an output. Includes a light path length adjuster to change the effective light path between the light paths:

A first polarization switch for selecting a polarization state for the input beam on the input optical path; And

Having birefringence properties and thus defining at least two possible effective optical paths of different lengths, for passing the input beam along the selected one of the at least two possible optical paths according to the selected polarization state of the input beam, and the light Means for modifying an effective optical path length comprising an optical element on the output path for providing an output beam of light traveled along a selected optical path

It provides a display device for generating a three-dimensional volumetric image, comprising.

According to another aspect, the present invention provides a method for generating a three-dimensional volumetric image,

Generating a two-dimensional image on the two-dimensional image display panel;

Projecting the two-dimensional image as a virtual image onto the imaging volume using the first focusing element; And

Display panel and projection focusing element to change the position of the virtual image within the imaging volume by changing the effective optical path length between the output optical path and the input optical path of the optical path length adjuster disposed between the display panel and the projection focusing element. Changing the optical path length between

Selecting a polarization state for the input beam of light on the input optical path using the first polarization switch;

Passing an input beam through an optical element having birefringence properties and thus defining at least two possible effective optical paths of different lengths, wherein the input beam is subject to the at least two possible effective according to the selected polarization state of the input beam. Passing through the optical device, moving along the selected one of the optical paths; And

Providing a light output beam from a birefringent optical element on said light output path

Including, changing the light path length

It provides a method for generating a three-dimensional volumetric image comprising a.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings.

1 is a perspective schematic diagram of a volumetric display based on two scanning lasers and an up-conversion crystal;

2 is a perspective schematic view of a volumetric display based on a switchable cell of a polymer sprayed liquid crystal or liquid crystal gel.

3 is a schematic diagram useful in explaining the principle of a volumetric three-dimensional image display device in which the present invention may be usefully deployed.

4 is a schematic diagram illustrating a volumetric three-dimensional image display device comprising a display panel and an optical path length adjuster in accordance with the present invention.

5 is a schematic diagram of a volumetric three-dimensional image display device using an optical path length adjuster between a display panel and a focusing element.

6 is a schematic diagram of an optical path length adjuster providing two different path lengths.

7 is a schematic diagram showing the effect of the orientation of the optical axis of a birefringent optical element on an input polarizing beam.

8 is a schematic diagram illustrating two different light paths of the regulator of FIG. 6.

9 is a schematic diagram of an optical path length adjuster providing two different path lengths based on a beam splitter and which may be used in conjunction with the adjuster of FIG.

FIG. 10 is a schematic diagram of a folded multi-path lightpath length adjuster that provides eight different lightpaths with seven different path lengths based on a beam splitter and may be used in conjunction with the adjuster of FIG.

FIG. 11 is a schematic diagram showing eight different light paths of the regulator of FIG. 10. FIG.

12 is a schematic functional block diagram of a control system for the display device of FIG.

FIG. 13 illustrates terms used to define a polar angle and an azimuthal angle with respect to the wave normal in a crystal of birefringent material.

FIG. 14 is a schematic diagram illustrating a convergent beam within a birefringent crystal and its various focusing points. FIG.

FIG. 15 is a graph showing the displacement of the focal point of FIG. 14 as a function of angle of incidence for a normal light beam and a special light beam having an azimuth angle of 0 degrees and 90 degrees.

16 is a schematic diagram of a cylindrical birefringent element associated with a non-birefringent compensation correspondence for correction for astigmatism.

FIG. 17 is a graph showing the displacement of the focal point with respect to the lens arrangement of FIG. 16 as a function of the angle of incidence for the normal beam and each of the special rays having azimuth angles of 0 and 90 degrees.

18A and 18B are schematic views of a spherical birefringent element, which can be used in an optical path length adjuster with small astigmatism.

FIG. 19 is a graph showing image distances for each of the normal light beam according to FIG. 18B, the special light beam having an azimuth angle of 0 degrees, and the special light beam having an azimuth angle of 90 degrees. FIG.

20A and 20B are schematic diagrams of optical elements for correcting spherical aberrations in birefringent elements.

FIG. 21 shows the focusing point of a normal light beam and the focusing point of a special light beam having (i) an azimuth angle of 0 degrees, and (ii) an azimuth angle of 90 degrees, for a cylindrically corrected planar plate element as shown in FIG. 16. Graph showing the distance between the focus points of special rays.

FIG. 22 is a graph showing the image distance difference between a special image and a normal image for a special ray having an azimuth angle of 0 degrees and 90 degrees with respect to a spherical birefringent lens as shown in FIG.

3A and 3B illustrate some basic principles used in a three-dimensional image display device. In FIG. 3A, a relatively small virtual image 30 of the small display panel 31 is provided by Fresnel mirror 32. In FIG. 3B, a relatively small virtual image 35 of the small display panel 36 is provided by Fresnel lens 37. The virtual image 30 or 35 appears in the air in front of the lens. The observer can focus on the image 30 or 35 and observe that it is 'floating' in the air.

4A and 4B show a variant of the arrangement of FIGS. 3A and 3B. As shown in FIG. 4A, the effective optical path length between the display panel 41 and the Fresnel mirror 42 is changed by providing a suitable effective path length adjuster 43. Similarly, as shown in FIG. 4B, the effective optical path length between the display panel 46 and the Fresnel lens 47 is changed by providing a suitable effective path length adjuster 48.

In one embodiment, which is the subject of a separate patent application entitled "Volume measurement display" filed simultaneously by the same applicant, the effective path length adjusters 43 and 48 are variable intensity lenses; In other arrangements within the same application, an effective path length adjuster is a mechanically driven device that switches between two or more light paths in the physical movement of one or more optical elements.

In another arrangement, which is the subject of a separate patent application entitled "Optical path length adjuster" filed simultaneously by the same applicant, effective path length adjustment is performed electro-optically using a polarizing switch and a pair of beam splitters. The beam splitters are arranged to provide at least two different optical path lengths between them, which paths can be selected by polarization switches.

The present invention, however, relates to electro-optical switching between two or more optical paths in a birefringent optical element.

In a general sense, mirror 42 or lens 47 is generally used for projecting two-dimensional images of display panels 41 and 46 onto virtual images 40 or 45 located within imaging volume 44 or 49. It can be replaced or implemented by the optical focusing element of. Preferably, mirror 42 or lens 47 is a single or complex optical focusing element with a single focusing length such that the flat display panel is imaged on a single plane of the imaging volume.

5 shows the basic components of a display device 50 according to the principle of FIG. 4. The two-dimensional display device or 'light engine' 51 provides an illumination source for imaging in the image plane 55. Light is focused along the input optical path 52 to the optical path length adjuster 53 and the optical path length adjuster 53 through the output optical path 54 to focus the two-dimensional image onto the plane 55 ( 57 (eg, the mirror 42 or the lens 47).

The operation of the light path length adjuster 53 effectively moves the depth position of the image plane 55 as indicated by arrow 58. The path length is preferably adjusted periodically at the 3D image display frame frequency. Typically this will be 50 or 60 Hz. Referring again to FIG. 4, during the 3D image frame period (eg 1/50 second), the virtual image of the display panel 41 or 46 fills the imaging volume 44 or 49. Within the same frame period, the display panel can be driven to change the projected image, so that different depths in the imaging volume 44 or 49 will receive different virtual images.

In a preferred aspect, path length adjuster 53 is effective to periodically sweep a substantially planar virtual image of a substantially planar two-dimensional display panel through imaging volume 44 or 49 at 3D frame rate. Within that 3D frame period, the 2D image display panel continuously displays the 2D image at a 2D frame rate substantially higher than the 3D frame rate.

Therefore, in other planes 40a, 40b or 45a, 45b in imaging volumes 40, 45, different images are obtained such that a three-dimensional image of any object can be constructed.

The two-dimensional display panel can be any suitable display device for generating two-dimensional images. For example, this could be a projection display based on a poly-LED display, LCD, LCOS display or digital micromirror device (DMD).

Preferably, the display panel is fast enough to allow generation of a plurality of 2D images, for example within one frame period of 1/51 second. For example, commercially available DMDs can reach 10,000 frame rates per second. If 24 2d frames are used to generate color and gray-scale effects and a 3D image refresh rate of 50 Hz is required, eight different image planes 40a, 40b, 45a, 45b are created within the imaging volumes 44,49. It is possible to do

Referring to Fig. 6, a first arrangement of the optical path length adjuster 53a is described. The optical path length adjuster is based on the birefringent material and the polarization switch.

Birefringent materials have different effective refractive indices depending on the polarization of the light striking the material. This difference can be significant. For example, calcite, a well-known material, has n _e = 1.486 for light with polarization parallel to the optical axis of the material and n ₀ = 1.658 for light with polarization perpendicular to the optical axis (orthogonal). Has a refractive index of. The present invention is based on this attribute.

6 illustrates this principle. The optical path length adjuster 150 includes a polarization switch 160 in the input optical path 52 in front of the optical element 161 exhibiting birefringence properties. The light output path 54 is indicated from the output surface of the birefringent optical element 161.

The expression 'polarization switch' is used herein to include any device suitable for selecting a particular polarization state, such as a polarization rotator that can be switched on / off or switched from / to an optical path / optical path. do. The polarization switch can change the polarization state of an already polarized beam or can select the polarization state from an unpolarized beam. If the light from the display panels 41 and 51 is already polarized, the polarization switches can all be of the polarization change type.

The polarization switch can be a single cell liquid crystal panel with a twisted nematic 90 degree structure or a ferro-electric effect cell allowing higher switching speeds. Polarization switches generally provide polarized light output in one of two possible polarization states, depending on the applied electric field. In another alternative, the polarization switch can be implemented using a rotatable wheel with two alternative polarizers.

The expression “birefringent optical element” 161 is used herein to mean an optical element that exhibits sufficient birefringence properties to enable the selection of at least two different effective optical path lengths by the selection of the polarization state of the incident light beam. . The birefringent optical element can include focusing properties. The birefringent optical element can include portions that do not show birefringence, as discussed below.

Due to the different refractive indices n ₀ and n _{e in the} birefringent optical element 161, the apparent (effective) optical path length through it is more optical for light polarized in a direction 164 parallel to the optical axis 163. Longer for light polarized in a direction 162 perpendicular to 163 (or vice versa, depending on the crystal material). By switching the polarization switch 160 to select an appropriate polarization state, a short or long light path can be selected.

Care should be taken in selecting the direction of the optical axis of the birefringent optical element. The effective refractive index for the P-polarized light may vary depending on the angle of incidence. This can be inconvenient in an imaging system. In a typical application, the birefringent element 161 can emit light at various angles of incidence. Preferably the deformation of the index of effective refraction for light with one polarization (ie normal light or special light) should be minimized. This may be accomplished by selecting the optical axis of the crystal perpendicular to the optical axis of the system (ie, orthogonal to the input path 52, as shown in FIG. 6).

In this situation, for polarized light perpendicular to the optical axis, the index of refraction of the birefringent element 161 is equal to the index of the general refraction n ₀ of the birefringent element. For polarized light parallel to the optical axis 163 of the birefringent element 161, this situation is more complicated, as discussed in connection with FIG. 7.

7A and 7C show an overview of the birefringent optical element 161 in the direction parallel to the optical axis 163. 7B and 7D show the birefringent optical element 161 in the vertical direction with respect to the optical axis 163. The index of refraction depends on the direction of light transmission. Two values of the refractive index for the input beam polarization parallel to the optical axis are extreme values. For other propagation directions with the same angle θ _e , the refractive index has a value between these two values.

An actual embodiment is shown in FIG. 8. In this schematic diagram, the 'object' may correspond to a display panel light engine 51 that emits light along the input optical path 52 or to the birefringent optical element 161. Polarization switch 160 selects the desired polarization state for the input beam. The position of the images 55, 55 'depends on the polarization of the selected light.

An embodiment such as that shown in FIG. 8 may generally produce an image in only two different planes 55, 55 ′. A series (N) of these optical path length adjusters, each having a birefringent optical element 161 having a thickness twice as large as the previous birefringent element, will effectively result in a different path length of ^2N . For example, using eight polarization switches 160 and eight birefringent optical elements 161, where the polarization of the light rays is independently selected through the birefringent optical component, and the astigmatism problem will be avoided as will be discussed later. If possible or corrected, 256 different image planes can be realized.

The invention may also be used in conjunction with the lightpath length adjuster described in the co-pending application named “Lightpath Length Adjuster” as referenced above, and as discussed briefly below in connection with FIG. 9.

The optical path length adjuster of FIG. 9 includes a first polarization beam splitter 61 and a second polarization beam splitter 62. The polarization switch 60 is provided in front of the first beam splitter 61 in the input light path 52.

The first beam splitter 61 has a first input surface 61a and first and second output surfaces 61b and 61c, respectively. The second beam splitter 62 has first and second input surfaces 62a and 62b and an output surface 62c. The first optical path 63 extends directly between the first output surface 61b of the first beam splitter 61 and the first input surface 62a of the second beam splitter. The second optical path 64 (longer than the first optical path 63) is the second output surface 61c of the first beam splitter 61 and the second input surface 62b of the second beam splitter 62. Extend directly between them. The output surface 62c of the second beam splitter is connected to the output light path 54.

By the polarization switch 60, it is possible to select from two different optical paths 63 and 64 as follows. Assume, for example, having a polarization state P and starting with an input beam of light polarized on the input path 52. Two other paths 63 and 64 can be selected as follows. First, when the polarization switch 60 is switched off, the P-polarized light enters the first divider 61 and is not reflected here, but passes directly through the path 63. The same condition applies to the second divider 62. Thus, in this polarization state, light travels through the regulator 53a along a straight line.

When the polarization switch 60 is switched on, the P-polarized input light beam will be converted to S-polarized light. This polarization will enter the first divider 61 and will be reflected to the right of the light path 64. In the second divider 62 this light will be reflected again and out of the regulator 53a along the output path 54.

In the configuration of FIG. 9, it will be noted that the second light path 64 includes three path sections 64a, 64b, 64c separated by two mirrors 66a, 66b. In other arrangements, there may be more or fewer path segments.

With this adjuster 53a it is possible to create two image planes 55 in the volumetric display device 50.

Using the regulator 53a in conjunction with the birefringence regulator 150 of the present invention is possible to increase the number of image planes.

A more sophisticated path length adjuster 100 using the principles of the arrangement of FIG. 9 is shown in FIG. 10. With four polarization switches 101, 102, 103, 104 and only two polarization beam splitters 105, 106, the number of different light paths can be increased to seven. This is an advantageous configuration, especially since large polarizing beam splitters are relatively expensive.

Similar to the arrangement of FIG. 9, the input lightpath 52 is directed towards the first input surface 105a of the first beam splitter 105. The output light path 54 is connected to the first output surface 106c of the second beam splitter 106.

The first beam splitter 105 has first and second input surfaces 105a and 105d and first and second output surfaces 105b and 105c. The second beam splitter 106 has first and second input surfaces 106a and 106b and first and second output surfaces 106c and 106d. Arrays of mirrors 108a, 108b, 108c, and 108d fold the various lightpath sections to the appropriate input surface of the beam splitter as shown. The first light path 110 is between the output surface 105b and the input surface 106a. The second light path 111 is between the output surface 105c and the input surface 106b. The third light path 112 is between the output surface 106d and the input surface 105d. Each input surface 105a, 106b, 105d, 106a is associated with each of the polarization switches 101, 102, 103, 104.

In principle there are sixteen different states in which four polarization switches can be arranged. Many of these states actually allow the same path for the light beam to enter the regulator. It can be shown that there are eight different paths, and seven of these eight paths have different overall path lengths. Eight separate paths are shown in FIG. 11. Details of these routes can be found in the co-pending application referenced above.

The birefringent lightpath length adjuster 150 of the present invention may also be used in conjunction with the adjuster of FIG.

Other light paths may result in differences in brightness due to absorption coefficients of the polarization switches 60, 101-104 and / or birefringent elements 161 and / or dividers 61, 62, 105, 106. This absorption can be compensated for example by the intensity of the light engine display 51, which is electronically corrected in the video signal supplied to the light engine display 51.

Referring to FIG. 12, in conjunction with the control system, a schematic diagram of the entire volumetric image display device using the birefringent optical path length adjuster described herein is shown. The path length adjuster 120 inserted between the 2D display panel 46 and the focusing element 47 (eg, the adjusters 53, 150, 53a, 100 as previously described) is controlled by the path length control circuit 73. The path length control circuit provides a drive signal to each polarization switch The display driver 72 receives 2D frame image data from the image generator 71. The continuous display of the 2D image is performed by the synchronization circuit 74. Synchronized with path length controller operation.

The birefringent optical path length adjuster 150 described in connection with FIGS. 6, 7, and 8 generally suffers from aberration problems. Special rays are prone to astigmatism. Even when the angle θ _e is small (note that θ _e is defined relative to the optical axis of the system), the refraction by the crystal for two situations where the polarization of the beam is parallel to the optical axis of the crystal can be quite different, resulting in severe The astigmatism of the beam is focused through the birefringent plane parallel plates. This astigmatism results in a 'blur' of focus, which overlaps completely with normal focus. Thus, in many cases, optical path length adjustment cannot be performed usefully without providing correction of this astigmatism. There are several ways to correct these aberrations as described below.

In addition, spherical aberration can be severe for converging beams moving through the plane-parallel plate. In the case of spherical aberration, calculations show that optimizing spherical aberration correction for normal beams can also result in a significant reduction in spherical aberration of the special beam.

In one arrangement, it is proposed to include a (non-birefringent) spherical aberration-correcting optical element in a light path that corrects spherical aberration in a normal beam. Even if rotation of polarized light is applied and some plane-parallel plates are passed by a special beam, this spherical aberration correction is sufficient when the angle of incidence is not too large.

Light propagation in the birefringent material is now described briefly with reference to FIG. 13. Following the reasons explained in p.806 of the seventh edition of the Principle of Lenses (CUP Publishing, 2001) by M. Born & E. Wolf, we begin with the Fresnel equation of the wave normal:

Where v _p , v _x , v _y , v _z are the phase velocities and propagation of the three fundamental velocities, and s _x , s _y , s _z are the components of the wave normal in the crystal. Assume that the optical axis is in the x-direction. In other words,

Where v _e is a special speed and v _o is a normal speed.

Substituting these expressions into equation 1:

Is derived and thus

이다.

to be.

)(w.r.t. x-축)을 특징으로 한다. 즉,The direction of the wave normal (v) in the crystal is polar (θ) (wrt z-axis), and azimuth (

(wrt x-axis). In other words,

이다.And

to be.

13 illustrates the geometric arrangement.

Substituting these equations into equation 3

or

이다.

to be.

As a result, the phase velocity

을 따른다.or

Follow.

Assume the normal of the crystal surface is in the z-direction.

Snell's law can then be derived as

Note that the azimuth is the same inside and outside the crystal. For normal rays, Snell's law clr

이고,

ego,

Where n ₀ is the general refractive index. For special rays, Snell's law

이고,

ego,

Where n _e is the special refractive index. This equation can be solved for θ, resulting in:

This equation can be used to calculate the direction of the special wave normal within the crystal as a function of the wave normal outside the crystal. This can be rewritten in the form of Snell's law as follows:

This equation shows important properties: The effective refractive index depends on the angle of incidence and the azimuth of the incident wave.

는 다음과 같이 작성될 수 있다:Now calculate the effect of birefringence on the refracted rays. Wave vector

Can be written as:

에 대한 다음 식을 유도할 수 있다. Using

The following equation can be derived.

이 단위 벡터이므로, 이 식은 다음과 같이 재작성될 수 있다.

Since this is a unit vector, this expression can be rewritten as

This can be rewritten as:

And

와 광선 벡터(=그룹 속도 벡터)

는 다음 방법으로 관련된다[J.Opt.Soc.Am.A 제 19권, 제 5호, p.814(1992)]Wave vector

And ray vector (= group velocity vector)

Is related in the following way [J.Opt.Soc.Am.A Vol. 19, No. 5, p.814 (1992)].

즉,

In other words,

The result is:

Note that the direction of the light beam is different from the direction of the wave normal.

)은 다음과 같다.Angle of refracted rays with surface normals (

)Is as follows.

If you use this expression instead of s _x and s _y , you get the following result:

함수로서의 결과식은 다음과 같다.

The result expression as a function is as follows.

=90°) 내의 특수 광선의 경우, 이 식은 다음과 같다:yz-plane (

For special rays within (90 °), this equation is:

=0°) 내의 특수 광선의 경우, 이 식은 다음과 같다:On the other hand, xy-plane (

For special rays within 0 °), this equation is:

In the case of ordinary rays, the corresponding equation is:

The difference between the normal beam and the special beam can be used to change the position of the focal point of the converging beam. A disadvantage of birefringent crystals is the astigmatism in special rays, i.e. the difference in the refracted ray direction with respect to the rays in the xz-plane and yz-plane. 14 shows a converging beam 140 in a calcite crystal 141. Middle line 142 represents a normal ray. Outer line 143 and inner line 144 represent special rays in each of the xz-plane and yz-plane. For clarity, all special rays are actually shown within the same plane of the figure.

It is evident in FIG. 14 that the position of the focal point 145 can be changed as produced by the converging beam 140. However, switching from a normal beam to a special beam produces astigmatism that can completely overlap the general focus.

The equation for the distance between the focal point 146 without a birefringent crystal (indicated by the dotted line) and the focal point 145 with a birefringent crystal is as follows.

d is the thickness of the crystal 141. From this equation, it is clear that the spherical aberration is also introduced by the crystal since the 'focal distance' δ is a function of the angle of incidence. FIG. 15 shows the displacement of the focal point as a function of the angle of incidence for special and normal beams with azimuth angles 0 and 90 degrees for calcite crystals (n _o = 1.4864 and n _e = 1.6584) 10 mm thick. Line 182 shows the focal displacement for a normal ray with an azimuth of zero degrees. Line 183 represents the focal displacement for a special light ray with an azimuth angle of zero degrees. Line 181 represents the focal displacement for the special ray with an azimuth of 90 degrees.

In the first arrangement, astigmatism can be corrected by adding anamorphic optical power to the birefringent optical element 141 or 161. A suitable arrangement is shown in FIG. 16 and one of the surfaces of the birefringent element 165 has a circular shape with a suitable, non-birefringent corresponding element 166 attached thereto. The index of refraction of the corresponding element 166 should match the general index of refraction of the birefringent element 165. The normal rays are then unaffected by the curved surface, while the focal points of the special rays in both planes can coincide.

To simulate this principle, it is assumed that an incoming special ray in the xz-plane to be defocused before the birefringent crystal is reached. For paraxial rays, a certain out-of-focus condition chosen is assumed to match the focus of the special ray. 17 shows the resulting focal displacement as a function of angle of incidence. Curve 186 shows the focal displacement for a normal ray with zero azimuth. Curve 184 represents the focal displacement of the special ray with an azimuth angle of zero degrees. Curve 185 represents the focal displacement for a special ray with an azimuth of 90 degrees.

Potential disadvantages of cylindrical lens systems 165 and 166 are their complexity and small focal shifts of approximately 0.7 mm for birefringent materials (eg calcite) of 10 mm thickness. Another potential disadvantage is the fact that astigmatism can only be corrected for the distance of a particular object. Astigmatism occurs when the birefringent optical element changes the object distance away from the position to be corrected.

In another embodiment, instead of using the planar-parallel birefringent element 161, the birefringent spherical lens 201 may be used as shown in FIG. 18A. In FIG. 18A, plano-convex birefringent lenses are used, although other configurations may be used. The optical axis of birefringent element material 201 is assumed to be parallel to the x-axis. Preferably, the spherical lens is configured such that the astigmatism of the special ray is minimized. A more detailed simulated view of the planar-convex spherical birefringent lens 201 is shown in FIG. 18B with the planar surface placed at y = 0, normal image at y = -38.0 mm, special focus y = -35.9 mm, And the original object (virtual) is placed at y = -44.9 mm. Special rays are represented by line 208 and normal rays are represented by line 209. Note that the focus of these special rays overlaps.

Using the theory given above, the focal length of the lens 201 can be calculated. Lens 201 produces a difference in focus for two special rays. However, the calculation indicates that for the distance (s _o ) of the particular object, and for the cone of the particular ray 202, there is no astigmatism. 19 shows this principle for a planar-convex calcite lens 201 with a thickness of 5 mm, a radius of curvature of 100 mm, and an object disposed at y = -44.9 mm. In this figure, the image by using the distance (s _i) is a thin lens approximation, is shown for normal light and special light. Curve 190 represents image distance as a function of angle of incidence for a normal light ray with an azimuth of zero degrees. Curve 192 represents the image distance as a function of the angle of incidence for a special ray with an azimuth of zero degrees. Curve 191 represents the image distance as a function of the angle of incidence for a special ray with an azimuth of 90 degrees.

It is clear from FIG. 19 that the two special light ray curves intersect at an angle of incidence of 15 degrees. This angle of incidence can be tuned by selecting the appropriate object 51 distance and / or the shape of the birefringent lens (eg, the radius of the curve). In other words, the polar angle at which the two special rays have a common image can be given a value other than 15 degrees by changing the object distance or the lens shape. Thus, astigmatism can be minimized by selecting an appropriate optimum object distance for the display 51.

The image is shifted approximately 2mm when changing from normal to special. Note that next to the spherical lens, an aspherical lens can also be used.

A disadvantage of these methods for correcting or eliminating astigmatism is the fact that the best fit for minimum astigmatism depends on the object distance. This means that when a series of birefringent path length adjusters are used, the independent use of a separate adjuster can change the focus of the beam as shown by subsequent adjusters in the series. This can in particular limit the switching modes that can be used for birefringent plane-parallel plates, in which case the astigmatism can be greater than the change of focus from the normal light beam to the special light beam.

In the case of the birefringent spherical lens described above, this problem is less severe because the astigmatism is much smaller than the image distance.

In a series of birefringent lenses, the lenses can be switched independently. Care should be taken to ensure that the optical object distance and the image distance of the adjacent lens match at least as closely as possible to the normal light beam. This means that the image produced by the first lens is near the position of the optimal object, such as the second lens. This structure results in 2 ^N image distances for the N birefringent lens.

A possible switching scheme for birefringent plane-parallel plates is that all path length regulators are switched to normal mode except for one single regulator passed using a special mode. Changing the regulator passed in a special mode will change the optical path length. This structure produces N image distances for the N birefringent plane-parallel plates.

Spherical aberrations are still evident in FIGS. 17 and 19, especially at larger incidence angles. It can be seen that the shapes of the three curves in these figures are similar. Therefore, compensating spherical aberration for a normal ray compensates for a large part of the spherical aberration of the special ray, especially for relatively small incident angles.

Thus, in a more preferred arrangement, the birefringent path length adjuster includes a spherical aberration correcting element and a birefringent element, as shown in FIGS. 20A and 20B. In the first arrangement of FIG. 20A, the non-birefringent lens element 203 produces spherical aberration (as described above with respect to FIG. 16) that compensates for being introduced by the planar parallel birefringent element 204, which has been cylindrically corrected. It is introduced to. In the second arrangement of FIG. 20B, the planar-parallel, non-birefringent element 205 generates spherical aberration to compensate for the introduction by the birefringent spherical lens 206 (as described above with respect to FIGS. 18A and 18B). It is introduced to.

The two elements 205 and 206 can be combined by mounting them together. The two elements 203 and 204 can be combined by mounting them together or by integrally forming the spherical lens 203 with the non-birefringent portion of the element 204 that has been cylindrically corrected. For normal light beams and special light beams, spherical aberration is sufficiently corrected.

In FIG. 15, it is evident that for the external light beam (with the largest angle of incidence), the focal displacement generated by the birefringent plane-parallel element 161 is greater than for the internal light beam (with the smaller angle of incidence). . This is the opposite of the situation where spherical lenses created focus. Therefore, as shown by way of example in FIG. 19, the focus of the outer light is closer to the lens than the focus of the inner light. Therefore, non-birefringent or birefringent spherical lenses (eg, lens 201) can be used to compensate for spherical aberration of normal light rays.

In FIG. 19, the outer light is focused closer to the lens than the inner light, as is normal with a spherical lens. This spherical aberration can be corrected (partially) by adding a plane-parallel plate in the converging beam.

For both cases, the aspheric surface can also correct spherical aberration.

)(이하 '내부-초점 거리(inter-focus distance)'라 함)를 도시한다. 곡선(210)은 0도의 방위각을 가진 특수 광선에 대해 내부-초점 거리를 나타낸다. 곡선(211)은 90도의 방위각을 가진 특수 광선에 대해 내부-초점 거리를 나타낸다.FIG. 21 shows the distance between the general focus and the special focus for the planar-parallel birefringent element 161 (FIGS. 8 and 14) as a function of the angle of incidence.

) (Hereinafter referred to as 'inter-focus distance'). Curve 210 represents the in-focus distance for a special ray with an azimuth of zero degrees. Curve 211 represents the in-focus distance for a special ray with an azimuth of 90 degrees.

Assuming that the spherical aberration correcting element corrects all spherical aberrations of the normal ray, the variation of δ as a function of θ in FIG. 21 is the magnitude of the spherical aberration of the special ray. Up to an angle of incidence of 10 degrees, these aberrations are rather small. At an angle of incidence of 10 degrees, the focal point shifts 0.007 mm and 0.002 mm for azimuth angles of 0 and 90 degrees, respectively.

22 shows the image distance difference between the special image and the general image. Curve 220 shows the difference in image distance Δs _i between the normal ray image and the special ray image, where the special ray has a zero degree azimuth angle. Curve 221 represents the difference in the image distance Δs _i between the normal ray image and the special ray image with the special ray having an azimuth angle of 90 degrees.

It is clear that special rays with azimuth angle φ = 90 still suffer from spherical aberration. However, by tuning the intersection of two special curves 220,221, this spherical aberration can be minimized. For example, as described above, for a birefringent lens there is a combination of the distance of the object, the lens shape (e.g., the radius of the thickness and curve) and the angle of incidence, which means that any astigmatism of the ray with this angle of incidence and the object distance Does not cause. By changing the object distance or the lens shape, the incident angle at which astigmatism does not occur changes.

In sum, the present disclosure proposes to use a birefringent light component to adjust the optical path length. Astigmatism is a serious problem of such light components. As described herein, this can be minimized by using cylindrically corrected planar parallel plates or spherical birefringent lenses.

Also disclosed is a method for correcting spherical aberration of a birefringent element. Correcting the spherical aberration for the normal light beam in the birefringent plane-parallel device can also sufficiently correct the aberration of the special light beam when the incident angle is not too large. The result is an aberration corrected optical path length adjuster, which introduces only a small aberration in each 'switching state'.

Although the main and important use for the path length regulator as described herein is the application of a volumetric three-dimensional image display device, it will be appreciated that the regulator can be used in other optical means and devices, in which case two optical elements It is necessary or desirable to facilitate electro-optical switching of the optical path length between. This arrangement avoids the need for moving parts since the path length can be changed with each polarization switch via an electronic control signal.

Although various optical techniques have been described for correcting or minimizing aberrations introduced by a particular configuration of a birefringent optical path length adjuster, it is noted that correction or further correction of the aberrations may be electronically possible. For example, some correction may be made by making a change in the image displayed on the display device 51 as a function of whether the image is passed as a special light beam or a normal light (ie, as a function of the switching state of the polarization switch (s)). Can be done.

Other embodiments are intended to be within the scope of the appended claims.

The present invention relates to a method and apparatus for adjusting the optical path length between two optical elements, and is applicable to a display device for generating a three-dimensional volumetric image and a method for generating a three-dimensional volumetric image. .

Claims (26)

A display device for generating a three-dimensional volumetric image, Two-dimensional image display panels 41 and 46 for generating two-dimensional images; First focusing elements 42, 47 for projecting the two-dimensional image to the virtual images 40, 45 in the imaging volumes 44, 49; And Means (53, 120, 150) for changing the effective light path length between the first focusing element and the display panel projecting to change the position of the virtual image in the imaging volume, wherein the means for changing the effective light path length is an input light path ( A light path length adjuster for changing the effective light path length between 52 and the output light path 54, A first polarization switch 160 for selecting a polarization state for the input beam on the input light path 52; And Has a birefringence property and thus defines at least two possible effective light paths of different lengths, for passing the input beam along a selected one of the at least two possible light paths according to the selected polarization state of the input beam, and the light Optical elements 141, 161, 201 for providing a light output beam traveled along the selected light path on output path 54. Means for altering an effective light path length comprising a display device for generating a three-dimensional volumetric image. A display device according to claim 1, wherein the birefringent optical element (161) has an optical axis orthogonal to the optical axis defined by the input path (52) and the output path (54). The display device of claim 1, further comprising an optical element (165,201) for at least partially correcting for astigmatism. 4. A display device according to claim 3, wherein the birefringent optical element (165) comprises a cylindrical optical surface for correcting astigmatism. 5. A display device according to claim 4, wherein the birefringent optical element (165) further comprises a suitable, non-birefringent corresponding element (166) attached to the cylindrical light surface. 6. Display device according to claim 5, wherein the corresponding element (166) has an index of refraction substantially equal to the general index of refraction of the birefringent element (165). 4. A display device according to claim 3, wherein the birefringent optical element comprises a spherical lens (201). 8. A display device according to claim 7, wherein the spherical lens is a planar-convex lens (201). The display device of claim 1, further comprising an optical element for at least partially correcting spherical aberration. 10. A display device according to claim 9, wherein the birefringent optical element is a cylindrical-corrected plane-parallel plate and the spherical aberration correction element is a spherical lens. 10. A display device according to claim 9, wherein the birefringent optical element is a spherical lens and the spherical aberration correcting element is a planar-parallel plate. 12. The optical path length adjuster of claim 1, wherein the cascade formation is combined with at least one additional optical path length adjuster (53,150) of one of the preceding claims. 150. A display device for generating a three-dimensional volumetric image, wherein an output light path (54) of the 150 forms a continuous input path (52) of said additional light path length adjuster (53,150). 13. The optical path according to claim 12, wherein the optical paths of each of said optical path length adjusters (53,150) include different optical path lengths, so that a plurality of possible total optical path lengths are selected by appropriate selection of path lengths in each said optical path length adjuster. Display device for generating a three-dimensional volumetric image. The display device of claim 13, wherein each successive optical path length adjuster in series has a thickness of any other birefringent optical element and another birefringent optical element in the form of a staircase. The apparatus of claim 1, further comprising an additional light path length adjuster, wherein the additional light path length adjuster comprises: A first polarization switch 60 for selecting a polarization state for the input beam on the input light path 52; And At least two possible light paths 63,64,110,111,112 of different lengths, selected for passing the light beam along a selected one of the at least two possible light paths according to the selected polarization state of the input beam and on the light output path First and second beam splitters 61, 62, 105, 106 for providing light output beams that travel along the light path. And a display device for generating a three-dimensional volumetric image. 16. The light beam of claim 15, wherein the first beam splitter 105 receives the light from the light input of the first splitter according to the polarization state of the light at the light input of the first splitter. 105c) a first light input (105a) connected to the light output of the first polarization switch (101) for conversion to each; The second beam splitter 106 is optically connected to the first and second outputs 105b and 105c of the beam splitter 105 via the first and second optical paths 110 and 111, respectively. And a second light input (106a, 106b), wherein the second beam splitter (106) is arranged according to the polarization state of the light of the first and second inputs (106a, 106b) of the second beam splitter (106). Diverting light at the first and second inputs (106a, 106b) to first and second outputs (106c, 106d); The first output 106c of the second beam splitter 106 defines an optical output path 54, and the second output 106d of the second beam splitter 106 passes through the third optical path 112. Optically connected to a second input 105d of the one beam splitter 105; Each of the first, second, and third optical paths 110, 111, and 112 includes one of the second, third, and fourth polarization switches 104, 102, and 103. The first, second, third and fourth polarization switches are thus cumulatively coupled to at least one of the first, second and third optical paths between the input optical path 52 and the output optical path 54. A display device for generating a three-dimensional volumetric image, adapted to select cumulative combinations. 4. A display device according to claim 3, wherein the display panel (51) is arranged at a distance from the birefringent optical elements (141, 161, 201) such that astigmatism is substantially minimized or eliminated. 4. Display device according to claim 3, wherein the display panel (51) is arranged at a distance from the birefringent optical elements (141, 161, 201) such that spherical aberration is significantly minimized or eliminated. 10. The three-dimensional volumetric image of claim 9, wherein the display panel 51, the birefringent optical elements 141, 161, 201, and the spherical aberration correcting elements 203, 205 are relatively arranged such that the spherical aberration is substantially minimized or eliminated. Display device for As a method for generating a three-dimensional volumetric image, Generating a two-dimensional image on the two-dimensional image display panel (41,46); Projecting the two-dimensional image into a virtual image (40,45) in an imaging volume (44,49) with a first focusing element (42,47); And By changing the effective optical path length between the input optical path 52 and the output optical path 54 of the optical path length adjusters 53, 150, 120 disposed between the display panel and the projection focusing element, the position of the virtual image in the imaging volume is changed. Changing the optical path length between the display panel and the projection focusing element to change, Selecting a polarization state for an input beam of light on the input optical path using a first polarization switch (160); Passing the input beam through an optical element having a birefringence property and thus defining at least two possible effective optical paths of different lengths, the input beam being dependent upon the selected polarization state of the input beam. Passing the input beam moving along a selected one of the possible effective light paths And changing the optical path length comprising a three-dimensional volumetric image. 21. The method of claim 20, further comprising at least partially correcting astigmatism. 21. The method of claim 20, further comprising at least partially correcting spherical aberration. 21. The method of claim 20, wherein at least one additional light path such that the output light path 54 of the first light path length adjuster 150 forms a continuous input path 52 of the additional light path length adjusters 53,150. Passing the beam through a length adjuster, and selecting a lightpath length using each lightpath length adjuster. 21. The method of claim 20, further comprising positioning the optical path length adjuster at a distance from the object to be imaged to minimize astigmatism. 21. The method of claim 20, further comprising positioning the light path length adjuster relative to an object to be imaged to minimize spherical aberration. 24. The method of claim 23, further comprising selecting a different light path length in each said light path length adjuster (53a, 53b).

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